Power supply circuits utilized with X-ray tubes generally include controls for independently presetting anode kilovoltage and anode current flow prior to tube operation. Anode current flow is generally adjusted by presetting filament current to a predetermined value which is known to provide the required anode current flow at a selected anode kilovoltage. During application of the anode kilovoltage a feedback regulator may be utilized to control filament current in response to anode current. Such feedback regulators are, however, generally limited by the thermal time constants of the X-ray tube filament structure and do not, therefore, provide adequate regulation during the period immediately following the application of anode voltage to the tube.
Prior art X-ray generators included circuits for presetting the X-ray tube filament current to a value which had been determined, by previous measurement, to provide the preset anode current flow at the preset anode voltage. These prior art generators included a large number of calibration controls. During initial set up of the generator a separate control was adjusted to provide the necessary filament current for each of a large number of separate combinations of preset anode current and anode kilovoltage. Periodic readjustments of these controls were required as X-ray tubes aged or were replaced.
Many modern X-ray facilities include multiple X-ray tubes which are selectively powered from a common X-ray generator. X-ray generators which are operated in this mode required a separate set of filament current adjusting controls for each X-ray tube utilized.
Prior art circuits and methods which attempted to calculate filament current as a mathematical function of anode voltage, preset anode current, and operating time are known, for example, from U.S. Pat. No. 3,983,396, which is incorporated herein, by reference, as background material. Such circuits, however, failed to predict filament current with suitable accuracy and stability.